1. Field of the Invention
The present invention relates to a longitudinally coupled surface acoustic wave resonator filter which is, for example, used as a bandpass filter in mobile communication systems and particularly to a longitudinally coupled surface acoustic wave resonator filter including a quartz substrate.
2. Description of the Related Art
The filter in the intermediate-frequency stage (IF stage) of mobile communication systems is required to have wider band characteristics due to digitization of the communication systems. Furthermore, in addition to the ability to resist changes due to temperature and to obtain sharp attenuation outside the passband, unnecessary spurious output is also required to be reduced.
Up to now, among such IF bandpass filters, surface acoustic wave resonator filters including a quartz substrate are known. Among such resonator filters, there are a longitudinally coupled surface acoustic wave resonator filter in which a plurality of interdigital transducers (hereinafter referred to as IDTs) are disposed between two reflectors along the propagation direction of a surface acoustic wave and a transversally coupled surface acoustic wave resonator filter in which two IDTs are disposed in a direction that is perpendicular to the propagation direction of a surface acoustic wave.
The longitudinally coupled surface acoustic wave resonator filter has an advantage in that it is easy to increase the bandwidth than the transversally coupled surface acoustic wave resonator filter.
However, in the longitudinally coupled surface acoustic wave resonator filter, there is a problem in that many unnecessary spurious outputs are generated in the bandpass characteristics.
FIG. 8 shows the filtering characteristics of a related longitudinally coupled surface acoustic wave resonator filter constructed by using a quartz substrate. In this surface acoustic wave resonator filter, the passband is 246xc2x10.08 MHz, but, as shown by an arrow A, a large spurious output appears around 249.3 MHz or, and as shown by an arrow B, a spurious output also appears on the lower-frequency side of the passband on the opposite side of the spurious A. As a method for suppressing these spurious outputs A and B, so far the construction of multi-stage longitudinal connection is known. That is, it is known that the spurious outputs A and B can be suppressed by preparing a plurality of longitudinally coupled surface acoustic wave filters and having them longitudinally connected. However, in the construction of multi-stage longitudinal connection, there are various problems such as the increase of insertion loss, the increase in size of a piezoelectric substrate, a coil required for adjustment of the impedance between stages when the impedance of elements is capacitive, etc.
On the other hand, a method for suppressing spurious output by IDTs modified by weights can be also considered. Then, regarding the spurious outputs A and B, the distribution of effective current flowing through electrode fingers at each frequency is shown in FIGS. 9A and 9B. As clearly understood in FIG. 9B, in the case of the spurious output B, the portions in which the polarity of current is opposite to each other exists inside one IDT. Accordingly, it is understood that the current flowing inside one IDT can be canceled out by electrode fingers modified by weighting as in a method of thinning, etc.
However, as shown in FIG. 9A, at the frequency of spurious output A, only the current of the same polarity exists inside one IDT. Accordingly, it is very difficult to reduce the amplitude of the current at the frequency of spurious output A by modifying the current amplitude by weighting. That is, it is difficult to cancel out the above-described spurious output A by modification by weighting.
In order to overcome the problems described above, preferred embodiments of the present invention provide a longitudinally coupled surface acoustic wave resonator filter including a quartz substrate having excellent temperature stability in which the loss is greatly reduced and unnecessary spurious output is effectively prevented.
A longitudinally coupled surface acoustic wave resonator filter according to a preferred embodiment of the present invention includes a quartz substrate, a plurality of IDTs arranged along the propagation direction of a surface acoustic wave on the quartz substrate, and reflectors provided on both sides of an area in which the IDTs are provided in the propagation direction of a surface acoustic wave. In the longitudinally coupled surface acoustic wave resonator filter, the distance L1 between the centers of the electrode fingers, which are the closest to each other, of adjacent interdigital transducers satisfies equation (1):
(0.35+n/2)xcex less than L1 less than (0.55+n/2)xcexxe2x80x83xe2x80x83(1)
where n=0, 1, 2, 3 . . . , and
the distance L2 between the center of the innermost electrode fingers of the reflectors and the center of the outermost electrode fingers of the interdigital transducers adjacent to the reflectors satisfies equation (2):
(0.10+m/2)xcex less than L2 less than (0.40+m/2)xcexxe2x80x83xe2x80x83(2)
where m=0, 1, 2, 3 . . . .
A longitudinally coupled surface acoustic wave resonator filter according to another preferred embodiment of the present invention includes two sets of the longitudinally coupled surface acoustic wave resonator filter according to the above-described preferred embodiment of the present invention, the two sets of the filters have different characteristics from each other, and are provided on the same quartz substrate, and the inputs and outputs of the two filters are electrically connected in parallel, respectively.
In a longitudinally coupled surface acoustic wave resonator filter of preferred embodiments of the present invention, the plurality of IDTs constitute first and second IDTs.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.